We have previously experienced that at least 60% of the families do agree to participation of their children in research studies. The prospective participants of this study will be carefully examined and assessed by a physician before enrollment. A detailed history of their previous immunizations will be obtained, and their nutritional status and recent illness (as outlined below) that could compromise the immune system and family background etc. will be considered before study enrollment. The participants should be free from any chronic disease or any recent illness that may compromise the immune system including arsenic related skin lesions in children in Shahrasti where only Dukoral vaccine would be evaluated.